1. Field of the Invention
This invention relates to digital matched filters using Fast Fourier Transform devices.
2. Description of the Prior Art
Pulse compression filters or digital matched filters using Fast Fourier Transform (FFT) devices are well known in the prior art. Such filters are described in a paper entitled "Digital Matched Filters Using Fast Fourier Transforms" by H. M. Halpern and R. P. Perry in the EASCON 1971 Proceedings, pp. 220-230, published by the Institute of Electrical and Electronic Engineers, Inc.
In prior art systems, a stored linear frequency modulated (FM) reference signal is combined with a second linear FM signal, in a suitable mixer or signal multiplier unit. As an example, the second linear FM signal may be the intermediate frequency (IF) signal from a radar receiver. The resultant continuous wave (CW) mixer output signal, in the time domain is then processsed by a first Fast Fourier Transform device which transforms the time domain mixer output signal to a signal in the frequency domain. The frequency domain output signal from the FFT may then be further processed by additional system devices in order to meet desired system specifications with respect to a desired pulse compressed system output signal in the time domain.
The magnitude of the prior art mixer output signal is proportional to an arbitrary time, .DELTA.t, which is measured from the time an IF signal is coupled to a mixer input port relative to the time at which the reference signal is coupled to a different mixer input port. As the arbitrary time, .DELTA.t, of the IF signal coupling increases, the magnitude of the mixer output signal decreases resulting in a decrease in IF signal resolution. For many radar applications such a limitation is not acceptable.
See for example, an article entitled "A Technique for the Time-transformation of Signals and its Application to Directional Systems," by W. J. Caputi, published in "The Radio and Electronics Engineer," March, 1965, pages 135-142. This article describes a passive time-transformation technique, termed "stretch," for combining an input linearly FM signal with a reference linearly FM signal to generate a substantially constant frequency signal that can be arranged to manifest a pulse compression or expansion of the original input FM signal.